Biopolymer–Solvent Phase Behavior for the Lignin–Acetic Acid–Water System

Lignin is unique among renewable biopolymers in having significant aromatic character, giving it potential applications for polymeric systems ranging from electrodeposited coatings to polyurethane foams to an inexpensive precursor for carbon fibers. Thies and co-workers (2015) have developed a technique for simultaneously solvating, fractionating, and purifying the lignin polymer recovered from by-product streams of the lignocellulosic biofuels and pulp-and-paper industries. Called Aqueous Lignin Purification with Hot Acids, or ALPHA for short, the process involves combining solid lignin with hot acetic acid–water mixtures to produce two liquid phases: a highly solvated, polymer (lignin)-rich phase and a solvent-rich phase. Surprisingly, the polymer can be easily partitioned by molecular weight into either phase simply by varying the acetic acid–water ratio (e.g., from 20/80 to 80/20). In addition to partitioning by molecular weight and generating low polydispersities, ALPHA allows one to both solvate and “ultra-clean” the high mol wt fraction of the polymer so that it is clean enough (ppm metals levels) for conversion to carbon fiber. However, as of yet the thermodynamics responsible for the creation of this fascinating polymer–solvent system have yet to be examined. To obtain a better fundamental understanding of how and why the acetic acid–water solvent system is so effective for processing lignin, the compositions of the polymer-rich and solvent-rich phases in our system were measured, with an emphasis on the partitioning of the acetic acid and water between the two liquid phases. In particular, liquid–liquid equilibrium compositions of each phase were measured for a softwood-derived lignin with acetic acid–water mixtures at temperatures ranging from 65 to 95 °C. The global pseudoternary phase-behavior characteristics of this polymer–dual solvent system were also elucidated and will be discussed, along with the implications of the observed polymer–solvent phase behavior for the development of a viable purification and fractionation process for the renewable biopolymer lignin.